This invention relates to an antifouling coating composition containing the combination of metallic zinc and carbon for producing biocidal substances when contacted by the combination of dissolved oxygen and water. The invention also relates to a method of preventing fouling of a structure which may be or is contacted with water, such as buoys, floating piers, ship bottoms, collecting or discharge water passages to which pestiferous organisms attach themselves.
More specifically, this invention relates to an antifouling coating composition for a structure which may be or is contacted with fresh or seawater, said composition being composed of
(a) about 10 to 60% by weight metallic zinc powder, PA1 (b) about 5 to 40% by weight carbon, PA1 (c) the remainder a vehicle comprising film forming and pore forming materials.
There are two types of conventional antifouling coatings, classified according to their mode of action. They are generally known as "soluble matrix" or "contact" types.
In the soluble matrix type the toxicant is dispersed in a binder that is slightly soluble in water. The toxicant is slowly released as the binder dissolves. The rate of dissolution must be carefully controlled since inadequate concentration of poison at the surface will allow attachment of the pestiferous organisms. It is difficult in practice to control the rate of dissolution of the toxicant. In order to achieve long service lives with soluble matrix antifouling coatings, the antifouling agents must be extremely toxic. Typical coating compositions contain copper suboxide, tetramethylthiuram sulfide, zinc dimethyldithiocarbamate, tributyltin oxide, etc. as antifouling agents. The above-exemplified agents have known adverse effects on other living organisms. Soluble-matrix type coatings are now being banned because of the resulting secondary pollution to the aqueous environment.
The contact type of antifouling composition is formulated at a very high toxicant volume, such that the toxicant particles in the disperse phase are in contact with each other in the dry coating. Sixty percent by weight cuprous oxide is a typical toxicant because cupric ions are extremely toxic to aqueous life. In the case of barnacles about 10 ug of copper per cm.sup.2 per day is sufficient to prevent their attachment. The binder is largely insoluble in water, so that when the dissolution of the toxicant takes place a porous film of the coating binder remains on the surface. The binder is not entirely insoluble, and the balance between binder solubility/insolubility plays a critical role in controlling the rate of toxicant release. This is difficult to control, especially since various organisms exhibit different degrees of sensitivity to poison. Even if this problem could be solved, their remains the problem that the toxicants commonly used in contact type coatings persist and ultimately produce adverse effects on other living organisms in the aqueous environment.
I have made investigations in order to develop a new type of antifouling coating composition for the above-exemplified water-contacting structures, which can overcome the aforesaid troubles of secondary pollution which the conventional antifouling compositions essentially have. Consequently, I have succeeded in developing an antifouling coating which is quite free from any antifouling agent causing environmental pollution and can exhibit an excellent antifouling effect against pestiferous organisms.